Commercial dishwashers for which the invention can be used include program-controlled dishwashers (called batch dishwashers or batch warewashers or else box machines), conveyor dishwashers (conveyor warewashers) in the form of, for example, flight-type dishwashers (flight-type warewashers) or in the form of rack conveyor dishwashers (rack conveyor warewashers) and hood-type dishwashers (hood-type warewashers). The program-controlled dishwashers have a single treatment chamber for accommodating and for treating items to be washed. A dish rack into which the items to be washed can be loaded is preferably provided. The treatment chamber can be closed by a door. The conveyor dishwashers have at least one wash zone and at least one final rinse zone. Hood-type dishwashers have a treatment zone beneath a hood which is arranged such that the hood can move up and down.
A program-controlled dishwasher is known, for example, from DE 10 2005 023 429 A1. Conveyor dishwashers are known, for example, from WO 2006/007236 A2, EP 1 637 059 A2 and DE 10 2005 035 764 A1.
Items to be washed include, in particular, crockery, glasses, cutlery, cooking utensils, baking utensils and serving trays.
When cleaning items in commercial dishwashers, in particular in program-controlled dishwashers, it is often necessary to have cleaning programs with different liquid pressures, which are matched to the items to be washed, for the wash liquid and/or for the final rinse liquid.
If an electrical centrifugal pump whose polarity can be reversed and which can be changed over from two-pole operation to four-pole operation is used to deliver the wash liquid, a reduction in the rotational speed of the pump motor from 3000 revolutions per minute to 1500 revolutions per minute can be achieved on account of such polarity reversal. This reduction in the rotational speed causes a change in the water pressure at the pump output (pressure side of the pump). However, halving the rotational speed of the pump motor (and therefore also the rotational speed of the pump) in this way does not halve the water pressure. Instead, the water pressure changes from approximately 1.0 bar at 3000 revolutions per minute to approximately 0.25 bar at 1500 revolutions per minute. This disproportionate change means that the wash liquid pressure is often too low for glasses and crockery. A wash liquid pressure of approximately 0.5 bar would be more advantageous. The liquid pressure acting on the items to be washed is dependent on the line system, for example wash system and/or final rinse system, installed in the dishwasher.
It would be desirable to provide a way of alternatively generating in each case two different liquid pressures in a simple manner, with each liquid pressure being matched to the different items to be cleaned.